A hydrogen storage alloy is an alloy capable of safely and easily storing hydrogen as an energy source and has drawn an attention as a new energy conversion and storage material and its application fields are in a wide range, e.g., hydrogen storage and transportation, heat storage and transportation, heat-mechanical energy conversion, separation and purification of hydrogen, isolation of hydrogen isotopes, batteries using hydrogen as active materials, catalysts for synthetic chemical, temperature sensors, and so forth.
For example, a nickel-metal hydride battery using a hydrogen storage alloy as a negative electrode material has characteristics such as (a) high capacity; (b) durability to overcharge and over discharge; (c) capability of charging and discharging at high efficiency; (d) cleanness and has been actively investigated to have further improved capabilities (improvement of retention ratio of capacity in the case of repeating charge and discharge, that is, cycle life, improved capacity of the battery, etc.).
So far, an AB5 type rare earth-Ni-based alloy having a CaCu5 type crystal structure has been put into practical use as an electrode material for a nickel-metal hydride battery, one application example of such a hydrogen storage alloy; however the discharge capacity reaches almost a limit of about 300 mAh/g and presently it becomes difficult to further improve the capacity.
Further, as a new hydrogen storage alloy, a rare earth metal-Mg—Ni based alloy, for example, LaCaMgNi9 alloys (Patent Document 1) having a PuNi3 type crystal structure have drawn attention and it is reported that a discharge capacity exceeding that of an AB5 type alloy can be obtained by using these alloy for electrode materials.
It is also reported that in addition of the crystal phase having the AB5 type crystal structure, electrode materials of hydrogen storage alloys containing a crystal phase of AB2 type crystal structure such as MgCu2 type or rare earth metal-Mg—Ni type alloys containing, as a main phase, the crystal phase having Ce2Ni7 type, CeNi3 type, or Gd2Co7 type crystal structure have high hydrogen storage capacities and show good hydrogen release characteristics (Patent Document 2).
Furthermore, with respect to alloys having Ce5CO19 type crystal structure, it is reported that electrodes complexed with rare earth-Ni alloy having a CaCu5 type crystal structure are excellent in hydrogenation reaction speed (Patent Document 3).
Patent Document 1: Japanese Patent No. 3015885
Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 11-323469
Patent Document 3: Japanese Patent No. 3490871